These were pretty expensive, around 900 Euro which includes labor. That is only for the front. In an original Toyota part catalog I see that they are listed for 649 Euro. Maybe Lexus USA thought that these are to expensive and had a cheaper version made for the American market?
I read a review of the cheaper version, were the say that the illumination was uneven and it didn't look as good as door sills of other Lexusses. I just can talk about the ones I have, they really look fantastic and high class. I'll try to make a better image and post it.

The look different than the ones my Lexus dealer installed, and different than the ones oat the Lexus UK site.
Maybe there are different versions of the door-sills, or the US dealer is selling non-official parts. I don't know, I just see the difference.
Without more knowledge we can't say that Lexus UK is ripping you off. Here in the Netherlands the door-sills are expensive as well.